All the ones that I have made/used have leaked. For general protection (impact etc), home-made covers are pretty good, (my mum taught me how to cover books at school - but I don't use brown paper or flowery wallpaper offcuts now) - those document wallets found in office work well if you cut them to shape and use plenty of tape - duck tape is tough. Sticky backed plastic also works well. Leather looks nice and you can decorate with whatever you like (I like the simplicity of a tied leather thong/lace (bootlace or the like) to hold it together. The old trick is always to do one cover first, then close the book before measuring/cutting the second cover, to ensure there is enough slack - otherwise you give yourself a headache.
If you really don't want things to get wet, then try an Ortlieb map/document case. My passport is always in one. Not very diy, I know, but they work.